1. Field of the Invention
The present invention relates to an optical pickup that projects a laser beam onto an optical disc for reproducing or recording information. In particular, the present invention relates to an optical pickup having lenses with different numerical apertures for recording or reproducing information on a CD, a DVD and a BD.
2. Description of Related Art
An optical disc apparatus records or reproduce information on the optical disc as a recording medium by projecting a laser beam onto the optical disc from the optical pickup. If the optical disc apparatus supports a CD (Compact Disc) medium and a DVD (Digital Versatile Disc) medium, the optical pickup is equipped with a laser light source that emits infrared laser (having a wavelength of approximately 780 nm) for a CD and red laser (having a wavelength of approximately 650 nm) for a DVD medium. In addition, the optical pickup is equipped with an objective lens that condenses the laser beam to make a laser spot on a recording surface of the optical disc. A diameter of the laser spot is proportional to a wavelength of the laser beam and is inversely proportional to a numerical aperture (NA) of the objective lens. Since the CD medium and the DVD medium have different track widths, different objective lenses are necessary. However, a single objective lens can support both the CD medium and the DVD medium by modifying a shape of the objective lens.
In addition, the optical pickup detects reflection light from the optical disc so as to detect a position of the laser spot on the optical disc or read information. For this reason, the laser beam should be projected onto the optical disc so that an angle between the optical axis of the laser beam and the recording surface of the optical disc (tilt angle) is always a constant angle (right angle in many cases).
Therefore, in an invention described in JP-A-H7-57273, a lower surface of the objective lens facing a objective lens retaining portion is provided with a spherical seat, so that the objective lens can rotate in a plane crossing in the optical axis direction for adjusting an inclination (tilt angle) of the optical axis of the objective lens.
In a invention described in JP-A-S62-170028, the objective lens is fixed, and adjustment of inclination (tilt angle) of the optical axis of the objective lens is performed by adjusting inclination of the actuator with respect to the optical pickup utilizing a protrusion formed on the optical pickup main body and a spacer that is disposed between the actuator and the optical pickup as for the attachment angle of the actuator for driving the objective lens with respect to the optical pickup main body.
In a invention described in JP-A-2003-272198, the optical pickup includes an objective lens holder that holds an objective lens, a shaft portion that supports the objective lens holder in a slidable manner, an actuator base that supports the shaft portion, and a spring between the actuator base and a frame. An angle between the actuator base and the frame is adjusted by adjusting an adjustment screw that is provided to the actuator base, so that adjustment of inclination of the objective lens is performed.
Recently, in addition, a BD (Blu-Ray Disc) having larger storage capacity than a DVD has been available. Since recording density of a BD medium is higher than that of a DVD medium, a track width of a BD medium is much smaller than that of a DVD medium, so a laser spot for a BD medium should be small. Therefore, in order to perform recording and reproducing information on a BD medium, a laser light source that emits blue laser having smaller wavelength (approximately 405 nm) than the infrared laser and the red laser is used.
On the other hand, since a numerical aperture of the objective lens for a CD/DVD is too small to condense the blue laser appropriately for recording or reproducing information on a BD accurately. In addition, if the objective lens for a BD is used, the infrared laser and the red laser cannot be condensed appropriately. Therefore, there is proposed a lens holder for holding the objective lens to which two objective lenses having different numerical apertures for a CD/DVD and for a BD are disposed.
If the optical pickup having two objective lenses of different numerical apertures are used, optimal laser spots can be formed on the recording surfaces of optical discs having different track widths, so that recording and reproducing can be performed accurately.
In the case of the optical pickup having two different objective lenses as described above, tilt angles of the objective lenses can be different from each other due to manufacturing errors of the objective lenses or assembling errors when they are attached to the objective lens holder. In this state, if the optical pickup is adjusted so that optical axis of the laser beam becomes perpendicular to the recording surface of the optical disc with respect to one of the tilt angles of the objective lenses in accordance with the method described in JP-A-S62-170028 or JP-A-2003-272198, the optical axis of the laser beam going out from the other objective lens is not always perpendicular to the recording surface of the optical disc. It is because that a tilt error when the objective lens is attached to the lens holder or a shape error of a mounting portion is not always the same between the objective lenses.
Therefore, JP-A-H9-230111 discloses a structure in which two objective lenses are formed by integral molding. By forming the two objective lenses by integral molding, it is able to suppress generation of a tilt error between the objective lens and the lens holder, and a tilt angle error of the objective lenses due to a shape error of the attaching portion of the lens holder.
However, if the objective lenses having different numerical apertures are molded integrally like the invention described in JP-A-H9-230111, there may be a case where a malfunction occurs depending on a condition of the molding or a condition of a process after the molding, in which tilts of optical axes of the two objective lenses are different. In this case, since the two objective lenses are molded integrally, they cannot be adjusted separately with respect to their tilt angles. Therefore, it is necessary to improve accuracy of the molding, which causes increase of cost of manufacturing the lenses and the optical pickup.
In addition, it is possible to adjust each of the two objective lenses by utilizing a mechanism for adjusting the objective lens like the invention described in JP-A-H7-57273. However, if the adjustment mechanism for the objective lens is provided, spherical seats for the objective lens and the objective lens mounting portion should be formed. Therefore, manufacturing cost of the objective lens and the optical pickup may increase. In addition, since a plurality of adjustment mechanisms are necessary, a structure of the optical pickup becomes complicated, weight of the lens holder increases, and effort and time are necessary for assembling and adjustment.